1. Field of the Invention
The present invention generally relates to the measuring of various parameters of thin materials. More particularly, the present invention relates to a method and apparatus for moving an article relative to and between a pair of non-contact thickness measuring probes for the purpose of collecting data about the article which is moved in relation to the probes so that the thickness, bow and warp parameters of the article may be determined.
2. Description of the Prior Art
ASTM F1530 describes a standard test method for measuring flatness, thickness and thickness variations of semiconductor wafers by noncontact scanning. According to the ASTM test standard, the knowledge of flatness, thickness and thickness variations of semiconductor wafers is useful to both producer and consumer of such wafers to determine if the dimensional characteristics of a given wafer satisfy certain specified geometrical requirements.
Under the standard's test method, the above characteristics are measured by placing the wafer between two probes and measuring the distance between the surfaces of the wafer and the respective adjacent probes. More specifically, the wafer, supported by a chuck, is scanned along a prescribed pattern between both members of an opposed pair of probes to generate an array of displacement values. From knowledge of the array of displacement values, thickness, flatness and thickness variations can be determined. The method described in ASTM F1530 presumes that the wafer is held by a chuck having a face that is perpendicular to a measurement axis that is drawn between the two probes and that is ideally flat and clean.
U.S. Pat. No. 4,750,141 discloses a dual probe wafer thickness measurement gauge using capacitive sensors. The gauge uses a chuck to hold the wafer. The wafer is rotated and translated between the dual probes to develop an array of data. The gauge has a means for aligning the wafer so that the data points can be repeated whenever the wafer is turned over, as required to scan the entire wafer surface. Due to the method for holding and scanning the wafer, the wafer must be chucked in at least two non-overlapping portions of the wafer. The wafer measurements is repeated and the two maps stitched together. The repositioning of the wafer and the stitching of the maps increases the time required to characterize a wafer.
All dual probe measurement gauges experience distance dynamic range non-linearities that cause thickness errors as either or all wafer warp, wafer tilt in mount and mount distortions move the wafer nearer or farther from the probes during the scanning of the wafer between the probes. To overcome the non-linearity related errors, extensive calibration of the gauge prior to measurement is required and as well as the application of that calibration to each wafer computationally during a measurement of the wafer. The calibration and application thereof increases the time required to characterize a wafer.